1. An individual that is considered to be a newbie (newb), noobie (noob), or nub.
2. An individual that is new to something.
3. An individual that is not very good at something.
The new guy is a nubski, he has no idea what he's doing.
Wow, this guy is so bad at this game. What a nubski!
The foreskin of a nub (or noob, a person who is new and/or plays a game childlike.) Calling someone a nubskin suggests that they play so poorly, there are indeed the foreskin of the nub.
Wow, nubskin, way to run in there and aggro everyone in the room and makeus wipe.